The link from a mobile station to a basestation in 3G (third generation) CDMA (code division multiple access) systems is a multiple access radio channel that uses CDMA as the access scheme. In this scheme many users share the same radio channel and each user is uniquely identified by a spreading code.
In 3G CDMA, the mobile transmitter in the mobile station is used by the end user to access the network. It may provide voice only, data only, simultaneous voice and data, and location services for many different environments including indoor, low mobility, full mobility, and fixed wireless. To support the various end user services, the mobile transmitter divides its transmission content into four channels. One of these channels, namely the pilot channel, is used to manage the air interface resource and aid in signal detection. The remaining three channels are used to carry user data and to implement the air interface protocol.
The pilot channel carries pilot bits which have the characteristic that they are known at the receiver, and also carries PC (power control) bits which though unknown at the receiver are easily detectable at the receiver, while the remaining subchannels carry information which is unknown at the receiver.
It is a well known fact that due to the nature of CDMA, the signals transmitted by one user will occupy the same bandwidth as signals transmitted by other users and in fact all the signals of other users may be modelled as a form of interference. About 30% of the interference caused by other users is attributable to the pilot channels and 70% is attributable to the remaining information channels. Many interference cancellation schemes have been proposed for eliminating the entire contribution of other users. Such schemes are very complex because some sort of estimate of the received signal for other users including unknown components must be made.
Any substantial reduction in interference in a CDMA system is very important because this results in an effective increase in the system signal to noise ratio, and thereby allows a system to be deployed with fewer basestations. One of the most significant costs in deploying a CDMA network is simply that of obtaining rights to install basestations at the desired locations. Any decrease in the number of basestations required, even at the expense of a slightly increased cost per basestation, would be desirable.